Aluminum welding Q&A: About soot, porosity, and equipment

What causes soot and porosity in aluminum welding? Do I need a push/pull gun with my GMAW machine to weld aluminum? Are robots practical for my aluminum application? These questions are answered in this first installment of a new column on thefabricator.com.

Editor’s Note: This is the first installment in a recurring column that answers questions about aluminum welding. If you have a question about this process, e-mail your question to vickib@thefabricator.com. It may be answered in a future column.

Q: Why do I get so much black soot on and around my welds when I use 5356 GMAW wire to weld 6061T6?

A: The black soot actually is magnesium oxide. Magnesium content in 5356 is high (up to 5 percent). Some common base materials, such as 6061, also have relatively high amounts of magnesium—generally 1 percent.

When magnesium is vaporized by the arc’s heat and comes in contact with oxides, black soot forms. A proper gun push angle of approximately 15 degrees helps keep gas coverage at the leading edge of the weld pool, which reduces soot. Your shielding gas should be pure and moisture-free.

Clean the base material with a stainless steel wire brush, and use the correct weld parameters for the wire size to minimize overheating of the weld puddle. If strength is not the most important consideration, 4043 filler wire can be used to reduce soot in this application.

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Q: I have a small number of aluminum parts to weld. Do I need to buy a push/pull gun for my GMAW machine?

A: Not always. In most cases, a standard GMAW machine with the proper hardware can weld aluminum with very little difficulty. Your local welding supplier should have the following items that you can buy for a reasonable cost: U-groove drive rolls that prevent the wire from deforming during feeding; a Teflon® liner and guides to minimize drag through the system; and a shortened
gun conduit (10 ft. is a good length).

Keep the gun as straight as possible to optimize feedability. Some torch manufacturers make special contact tip sizes to allow for the expansion of the wire from the heat of the arc. Wire sizes of 3/64 and larger feed the best; any smaller and the feeding problems increase significantly.

Q: My company fabricates a variety of aluminum products, and sometimes we find porosity in the welds after cutting and etching. We have checked our argon gas shielding carefully and find it adequate. We have also consulted the aluminum material suppliers, both raw plate and sheet and filler wire. They all have recommended cleaning processes, which we now employ. However, we continue to find
porosity in our welds periodically. Is it possible the GMAW power source is influencing this contamination?

A: Unless the arc your present welding equipment generates is extremely erratic, you probably do not have a machine problem. Electricity generated in the arc is not contaminated. However, there is a type of pulse GMAW transfer available in some new generations of GMAW machines that can help eliminate this problem. This process superimposes a low-frequency pulse in combination
with the normal high-speed pulse frequency. Certain low frequencies actually agitate the molten weld puddle at a rate that allows typically trapped gases or impurities to escape.

This low-frequency process offers several additional advantages. Low-frequency weld puddle agitation has been found to reduce grain growth in aluminum materials, which minimizes weld cracking susceptibility. The most obvious benefit is the weld surface appearance. The lower frequency provides a weld bead that looks very similar to GTAW welding but at the speed of the GMAW process.

Check with power source manufacturers to find out which offer this process. Ask suppliers for data to confirm a reduction or elimination of porosity and reduction of crack susceptibility. Also request a demonstration to see if the machine provides the GTAW-like bead appearance.

Q: We manufacture high volumes of repetitive aluminum parts that we currently GMAW manually. Is it feasible to apply arc welding robots? We understand the care needed to feed aluminum GMAW wire by our own experience with manual equipment, especially the softer alloys like 4043.

A: Yes, it is feasible and done all the time, but because of wire feeding issues, especially in tight weld joint configurations, you should take precautions. As mentioned in the previous answer, do your homework and ask the robot supplier to demonstrate the machine’s abilities to GMAW aluminum.

Each manufacturer has its own recommendations and methods for properly feeding aluminum GMAW wire, but you can have success with both conventional robotic (aluminum) GMAW guns and more robust wire feeding systems that use servo-driven guns and assist wire feeders.